For a pve player like me, completing my pokedex is one of (if not the) most important goals to achieve.
When I say completing the dex, I'm talking about maximizing the caught, seen and evolution data, that currently is or ever has been viable to obtain.
With the option of trade chat, alot of grinding and a bit of luck (world quests, outbreaks), it is possible to maximize the caught and evolution data.
The problem lies in the fact, that currently there are seen datas, that are not obtainable anymore. I can think of hoopa and zygarde right now.
(Yveltal lost his spot in ash westbrooks team but suspicious bot still has it).
So my suggestion is to add them to the game somehow. An example would either be to add them to a boss team, do a new boss or even just do mini bosses for hoopa and zygarde, so players can actually complete their pokedex.